Satgen302  Satellite Hindsight No2   by GM4IHJ                 7th Jan 95
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It should not be forgotten in the light of their subsequent successes,
that the first two Uosats suffered traumatic failures shortly after
launch. Both were rescued after superb work by American Radio Amateurs
using a large Radio Astronomy antenna. Indeed legend has it , that a G4
visiting India was about to enter a temple , when he saw emerging from it
the Uosat team leader. " Have you been praying for your lost satellite ",
he asked. The reply he received is not on record. But, that very day Uosat
was reawoken by its command team, after they had received accurate orbit
tracking data from America. Tracking data from the silent satellite
derived by receiving its microwatt receiver local oscillator signal - not
its transmitter. What is perhaps noteworthy at this point is that, nothing
could have been done when the satellite was "found", if it had not been
designed with a secondary command line, which allowed access to the
satellite computer for subsequent control. So perhaps the real moral of
this tale is "that if you design into your satellite some emergency
control features, your God may well answer your prayers.
A different problem faced the Dove Oscar 17 control team when its
transmitter obstinately refused to shut down. Dove 2m Tx was programmed to
cycle down at regular intervals to allow ground control to uplink 2m
commands to it. So when the house keeping software latched up during
transmission there was no obvious way to get the Tx off and regain
control. Dove had no secondary command line . So the only way in was brute
force. Putting up a very powerful 2m signal , so that Dove could hear it
despite the 2m QRM from its own transmitter.
A big moonbounce array in Texas was tried , at first with no result. But
then an amateur in Italy pointed out that constant transmission was
flattening the Dove battery, so the Dove Tx was getting weaker. The point
being that one needed to get access when the Tx was weak enough to be over
powered but before the Dove battery got so flat that its Rx failed.
Eventually after much persistence Dove Tx was commanded off. Then after
allowing its battery to charge the inquest started. What to do next. Dove
onboard software was clearly corrupt and needed debugging or replacing .
But any new software sent up required acknowledgement by Dove.
Acknowledgement which could not come on 2m because once on the Tx would
stay on. Fortunately Dove had an S Band downlink. Unfortunately its PSK
keying was corrupted ( more about PSK later ). However an American radio
amateur with a big dish could just copy Dove on S band. So he relayed
details of Dove's state back to her controllers by landline telephone, and
eventually Dove was loaded with new software and back on the air. Software
which now carried a check to switch the Tx off if it latched up again. Eg
"On error.Tx off". If Dove had been designed with a secondary command line
( as Uosat ) it would not have been so difficult to sort out. Indeed
GM4IHJ still wonders where the Amsat controllers found the patience to
persist and fix it. Grateful thanks are certainly due to them.
PS Have been monitoring RS15 regularly. No trouble reading CW but USB is
difficult copy for IHJ ancient ears. The problem seems to be satellite
spin . With the transponder noise floor clearly audible as it drops nearly
10dB roughly ever 0.8 seconds. Lets hope the sat stabilizes or spin
reduces in the earths magnetic field due to electromagnetic braking.
73 de John GM4IHJ @ GB7SAN